The sources of the virulent strains of encephalitis arboviruses that produce epidemics and equine epizootics and the existence and location of these virus strains during interepidemic periods are major unkowns which currently impede progress toward prevention and control of epidemics of arboviral encephalitis. The aim of this project is to study the replication of epidemic-epizootic isolates of Venezuelan encephalitis virus in mosquito vectors and vertebrate amplifying hosts of enzootic habitats in order to learn if epidemic-epizootic strains can be maintained in cycles involving vector mosquitoes and vertebrate amplifying hosts of enzootic habitats. Two species of mosquitoes and two wild vertebrates from enzootic habitats will be employed to test selected epidemic-epizootic isolates of Venezuelan encephalitis for ability to cycle alternately between vector mosquitoes and vertebrate amplifying hosts. Both uncloned and cloned epidemic-epizootic viruses will be employed as well as cloned enzootic strains for control purposes. After cycling between mosquitoes and vertebrates, each virus strain will be analyzed for its marker characteristics and compared with original, unpassaged virus used to initiate the cycle. Six marker characteristics of epidemic-epizootic Venezuelan encephalitis virus strains will be utilized.